The 37-year-old Muslim convert was held for three-and-a-half years as an enemy combatant after his 2002 arrest amid accusations of plotting with al-Qaida to detonate a radioactive "dirty bomb."
In 2005, Padilla was linked to an existing Miami terrorism support case just as the U.S. Supreme Court was considering his challenge to President Bush's decision to hold him in custody indefinitely without charge. The "dirty bomb" charges were discarded and were never part of the criminal case.
U.S. District Judge Marcia Cooke imposed the sentence in Miami. Padilla, a former Chicago gang member with a long criminal record, had faced up to life in prison.
Going against federal sentencing guidelines and objections from federal prosecutors, Cooke said she was giving Padilla credit for his lengthy military detention at a Navy brig in South Carolina, the Associated Press reported.
"I dispute the government's contention that I can't take into account these considerations in fashioning a sentence,'' Cooke said.
She also agreed with defense lawyers that Padilla was subjected to "harsh conditions" and "extreme environmental stresses" while there.
"I do find that the conditions were so harsh for Mr. Padilla...they warrant consideration in the sentencing in this case," Cooke said.
Cooke also imposed prison terms on two other men of Middle Eastern origin who were convicted of conspiracy and material support charges along with Padilla in August. The three were part of a North American support cell for al-Qaida and other Islamic extremists around the world, prosecutors said.
Cooke sentenced Padilla's recruiter, 45-year-old Adham Amin Hassoun, to 15 years and eight months in prison and 46-year-old Kifah Wael Jayyousi to 12 years and eight months.
"I support the government's contention that these defendants were engaged in conspiracy to murder, maim and kidnap," Cooke said. "I want to send a message that these crimes will not be tolerated in this country and are not excusable.''
The men were convicted after a three-month trial based FBI telephone intercepts collected over an eight-year investigation.
"The main piece of evidence against [Padilla] was a form that he filled out back in 2000 to join the al-Farouq camp, which was one of the biggest and supposedly best in Afghanistan," AP reporter Curt Anderson told the NewsHour in August.
"Beyond that, there was very little other evidence against him. Most of it was the form which had his fingerprints on it, and that seemed to be enough to convince the jury that he had provided himself as material support to the al-Qaida terrorist group," Anderson said.